Whether from coal beds, oil fields, geopressured reservoirs, or other sources, production of natural gas (methane and other hydrocarbon gases) occurs in concentrated geological settings located mostly remote from the point of use. Increasingly, natural gas is shipped by ocean in liquefied form via tankers.
Compressors are used to increase the pressure of this gas and thereby reduce its volume for transport in these pipelines. The pressure also provides the motive force to move the gas down the pipe and to overcome friction losses along the way. Compressing this gas requires input energy, for a given quantity of gas, which is roughly equal to the product PΔV, where P is pressure and ΔV is the change in volume. Conversely, reducing the pressure of a gas under pressure via controlled expansion in a mechanical device recovers a portion of the energy which was previously invested during its compression.
Compressed gases exist in other processes as well, including those naturally pressurized at the wellhead such as carbon dioxide, helium, nitrogen and other gases, those compressed or generated at high pressure by industrial processes, including cryogenics, refrigeration, oil refining, chemical synthesis, and those compressed for the sole purpose of energy storage.
Regardless of the source of the pressurized gas, the need to reduce the pressure of the gas is typical. It also happens that the pressures needed to economically transport the gas over long distances are typically much higher than the pressures needed to distribute the gas locally, and also higher than typical users require. Currently, there are two common methods for reducing the pressure of the gas. The first method involves the use of a pressure reducing valve, regulator, or throttle, such an isenthalpic device that reduces the pressure. Unfortunately however the application of this method is undesirable because it causes the entropy of the gas to rise where this increase in entropy is irreversible, and represents a waste of energy. The second method uses a controlled expansion through a mechanical device, such as a turbine or a positive displacement mechanism, which extracts work from the gas. And by aspiring to an isentropic process, this second method recovers some of the input energy, leading to an overall higher process efficiency. Unfortunately, this second method also results in some undesirable consequences.
For example, regarding the use of a turbine, the controlled expansion of the gas via a turbine has several inherent problems. First, because the demand for gas varies with time and turbo-machinery has a narrow operating range for optimum efficiency, use of a turbine typically does not result in optimum efficiency. Second, gas can contain impurities which, due to the high differential velocity of the gas and the turbine rotor, can cause erosion and eventual failure of turbine components. Third, the price per watt of turbines under 1 MW in size increases substantially due in part to increasing speeds, and the need for gearboxes or frequency conversion electronics to couple their output to common power line frequencies of 50-60 Hertz.
Moreover, while expansion of the gas via a positive displacement mechanism effectively addresses the variable load, high differential velocity, and high speed issues, it has traditionally involved a sliding seal interface, such as a piston in a cylinder, or sliding vanes, scroll plates, or other wearing surfaces. This is undesirable because these types of systems typically have a limited operating life, introduce wear particles into the gas, or they may be excessively worn by particles already present and carried by the gas.
Thus, the application of energy recovery devices at gas pressure reducing stations has been limited.